Jim, what zone are you in?

THe Best estimate I could give you is about a 9b

or a cross between zones 10,11,12 Sunset

Sorry to bud in, but I remember Millet saying that SOIL temps have to be above 55* for growth. Air temps can get lower, I hear even lower than freezing, but may hurt some citrus ...like key lime. PPl in Ca & Carolinas have citrus inground & get frost, but soil temps don't get as low. If soil temps get lower than 55, trees will just go dormant & will start growth when soil temps go back up.

We have 35°F forcasted for this coming Thursday, but then a warm up afterwards. I don't want to bring the trees in this early if I don't have to. I can keep the root zone at above 55°F but unless I bring them in, the canopy will be a lot cooler.

i am sure alot of of you may disagree but i have a GH 35 by 96 totaly filled with citrus . i also have a deep well all left over from when i was in the nursery business. my irrigation is 5 rainbird sprinklers running in reverse to bust up the water moor . the cylinoid valve is attached to a thermostat the sole purpose of this is to keep the trees from freezing. it only comes on ten to twenty nights per year if i have plants in pots and dont want them to get too wet i just set them in an area that the sprinklers dont hit. this has worked well for me for about ten years . a friend is now using it as well. i have saw the sides iced over an inch or moor but i have never had a problem. the temp is usually around 50 . i have never had a problem producing a good crop it does get extremely hot on sunny days when we forget to drop the curtains. the only damage i have had was was minimal but keep in mind i am not trying to grow in a perfect environment i am just having fun and lots of it. our lowest temp.here is usually15 to 20.

Gina ,

I started a search and found this discussion on GW that might interest you.

http://forums2.gardenweb.com/forums/load/strucs/msg060302466879.html

There are lots of pictures to inspire you.

I just built a PVC greenhouse for about 500 dollars, and a friend did the electrical for free. It has been in use for about 8 weeks now. What would I change? I probably would make it an insulated one with the blower to inflate it. This would cut the heating expense to some degree. Just got my first electric bill, and the heating was about 140 dollars for 30 days. (Avg. nite temps of about 38 deg. to 50's.) Winds are a big consideration also. The framing needs to be sturdy.

The real issue here is to figure out what the main use and function of the structure will be and go from there. I had to decide that my house was going to be for tropicals and citrus and heat it accordingly. I wanted to do winter propagation of these plants and that means the air and soil has to be warm. You get the idea.

I would check thrift stores and salvage yards for recycled construction stuff. I purchased a really nice aluminum storm door for my gh for $10. I had to re-screen it but that was easy and cheap also. You may have to bite the bullet and buy a couple of pieces of lumber to frame it up and then attach the various windows, etc as to best fit.

I'll keep checking around as to any other ideas.

A few more thoughts,

Site the gh in a southern orientation and consider using solid wood on the north side of the gh. This would act as an insulator and protect against northern winds and save on the cost of windows. Fix the roof similar to the one in the link below and this would allow melting snow to drain to the northern side away from the front.

Check out this gh using recycled materials.

http://www.apartmenttherapy.com/la/look/look-greenhouse-made-from-salvaged-materials-061946